Thrust difference between A340 and 747

[Byrna]

Hello,
This is a question for all pilots and/or aircraft mechanics/engineers who work with the Jumbo Jet 747-400 and/or the Airbus A340 series aircraft.

I am a flight simmer who uses Microsoft Flight Simulator 2004 and recently noticed that the power rating of the engines of the A340-200/300 series aircraft is considerably less the engines of the 747-400 which surprised me. The gross weight difference between these aircraft is not that large - maybe 150,00 lbs. (representing a ratio of perhaps 1.4:1) but the ratio of the power (thrust) difference is literally 2:1 per engines - 31,000-34,000 lbs for the A340 and about 63,000 lbs per engine for the Boeing 747-400.

I am curious to know how Airbus manages to design the A340 so it can still travel at a cruise speed of mach 0.82-0.84. I figure there is some engineering design difference here from the 747-400 as the difference in GTOW weights is small when compared to the thrust diference.

John

[The African Dude]

Hi John,

The mass of the plane is not held up by the engines but by the wings, firstly! No no, stupid Andy, you know that...... *smacks forehead* but of course the engines give the wings the relative airflow for lift. So: I've heard one or two pilots mention that the different philosophy of design between A/B involves Airbus using "just enough" thrust to get off the ground but greater wing for cruising, and Boeing with lots and lots of thrust but "just enough" wing for cruising. Obviously I'm sure it's far more than "just enough" but you get the picture.

No idea how that works w.r.t. maintaining the same cruise speed though.

Andy

[Byrna]

Hi Andy,
What you say may explain the fact that the Airbuses (all models) glide so much better than the Boeings - the extra wing surface area! I mean with thrust at idle, they really "fly"! This may also explain the successful landing of that Airbus A330 by Air Transat which landed in the Azores back in 2001 by gliding down after the engines shut down due to fuel loss from a defective fuel pump installation-the pilot was a fellow Quebecois from my province in Canada!

I'd love to hear from others though to confirm what you said - especially pilots or engineers.

John

[747FOCAL]

It all comes down to:

Airbus, underpowering aircraft for 30 years..................:E


Maybe the A340-600 is different story, but the reat only fly cause the earth is round. :p

[MarkD]

As opposed to Boeing overpowering (see 757). Now Boeing are trying to replace that with 737-900X on ETOPS routes like YVR-HNL - good luck with that plan guys.

A342/3 were designed as frugal long range aircraft which made up time by not tech stopping. Bit of a hare and tortoise thing. Various threads here maintain 0.82-0.84 for 340 is a bit of a stretch in the real world.

The choice of CFM56 was either economic or franconomics (Snecma being a partner in CFM). The aforementioned RR 757 engines were probably a better fit (RB211-535C say) for power - especially with Trent 500s being the powerplant for the 345/6.

[Byrna]

Hi MarkD,

What do you mean when you refer to "tech stopping" - I'm not sure I follow?

Having said all this regarding underpowering the A340, does this represent any sort of safety hazard to the passangers and also, are there situations where this may also be a technical disadvantage to the better-powered 747's? (e.g. longer runways required for take-off perhaps).

John

[FakePilot]

Bryna,

Always be sure to take the sim with a grain of salt large enough to choke on. I used to form opinions of aircraft in the real world from the sim, until I discovered major discreapencies between the same airplane from two different software vendors. Duh! Should have known!

Also as Americans (you are, yes?) we have this stupid linear thinking programmed into us. This manifests in the general thinking that less of a good thing is bad and less of a bad thing is good, etc. The world doesn't always follow that model!

[MarkD]

Byrna

point being, a faster aircraft with less range would have to stop, while the slower but longer legged 340 cruises on overhead while the other plane is being refuelled. Introducing a 343 onto YVR-SYD has saved Air Canada a fuel stop southbound and this route is enjoying very healthy business.

As to disadvantage - most complaints about 340s seem to be about climb performance from ATC - a China Airlines 340 took off from an ANC taxiway so takeoff performance isn't so much of an issue :D With the 340 as with all 4 engined aircraft, you lose one engine means only 25% power down which is, in theory, safer than 3 or 2 engine equivalents, but no more so than 744. Safety-wise I don't recall any fatal incidents involving a 340 in 14 years revenue service, but I am open to correction.

[JetDriverWannabe]

As for the original difference between the A340 and B744 engine thrust.

The difference is mainly due big difference in total drag.

but, one must remember that the A340 has a much smaller capacity than the A340. Much much smaller. A B744 can carry 120 tons and has a mtow of 875,000lbs..A340-300 is only 41 tons and A340-600 only 55 tons..


Mtow on A340-300 is on 606,000lb ,A340-600 is 850,000 and uses 56,000lb engines.